Extension can not receive internal calls

Need help on this issue, thank you in advance first.

I use freepbx,version 2.10. Asterisk 1.8.

One of my extension can make outbound calls and receive inbound calls, but it can not be reached from internal phone.

When I try to dial the extension from other phones which are in the same network, it pops out the voice massage says" unavailable".

Could anyone help?